Method for operating a wind turbine with reduced blade fouling

ABSTRACT

A method of operating a wind turbine is provided. The wind turbine has at least one rotor blade and an active flow control (AFC) system. The at least one rotor blade has at least one aperture defined through a surface thereof, and the AFC system is configured to modify aerodynamic properties of the at least one rotor blade by ejecting gas through the at least one aperture. The method includes operating the wind turbine in a first mode, determining an environmental condition surrounding the wind turbine indicative of fouling of the AFC system, and operating the wind turbine in a second mode different than the first mode based on the environmental condition. The second mode facilitates reducing fouling of the AFC system.

CROSS-REFERENCE TO RELATED APPLICATIONS

Cross-reference is hereby made to related, commonly assigned, co-pendingapplications: docket number 235623 entitled “Active Flow Control Systemfor Wind Turbine,” docket number 235625 entitled “Systems and Methodsfor Assembling an Air Distribution System for Use in a Rotor Blade of aWind Turbine,” docket number 235850 entitled “Systems and Method forOperating a Wind Turbine Having Active Flow Control,” docket number235851 entitled “Apparatus and Method for Cleaning an Active FlowControl (AFC) System of a Wind Turbine,” docket number 235852 entitled“Systems and Method for Operating an Active Flow Control System,” docketnumber 235854 entitled “Systems and Method for Operating a Wind TurbineHaving Active Flow Control.” Each cross-referenced application isinvented by Jacob Johannes Nies and Wouter Haans and is filed on thesame day as this application. Each cross-referenced application ishereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present disclosure relates to methods of operating a wind turbinewith reduced fouling, particularly to wind turbines having an activeflow control (AFC) system, and a wind turbine including such AFC system.

Although horizontal axis wind turbines are well-established these days,there is still considerable engineering effort going on to furtherimprove their overall efficiency, robustness, and power generatingcapability.

This research has lead to the most recent AFC technologies which aim toimprove wind turbine efficiency. AFC technologies try to avoid flowseparation over rotor blades by actively modifying the wind flowproximate to the rotor blade. This can be achieved by ejecting gasthrough apertures formed in the surface of the rotor blade.

The introduction of such AFC systems has brought about the fact that theapertures used for blowing gas eventually collect dirt or impurities.This phenomenon is one aspect of what is referred to as blade fouling.Blade fouling can substantially lower the performance, in particular theextracted power of wind turbines.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, a method of operating a wind turbine is provided. Thewind turbine has at least one rotor blade and an active flow control(AFC) system. The at least one rotor blade has at least one aperturedefined through a surface thereof, and the AFC system is configured tomodify aerodynamic properties of the at least one rotor blade byejecting gas through the at least one aperture. The method includesoperating the wind turbine in a first mode, determining an environmentalcondition surrounding the wind turbine indicative of fouling of the AFCsystem, and operating the wind turbine in a second mode different thanthe first mode based on the environmental condition. The second modefacilitates reducing fouling of the AFC system.

In another aspect, a method of operating a wind turbine is provided. Thewind turbine has at least one rotor blade and an active flow control(AFC) system. The at least one rotor blade includes at least oneaperture defined through a surface of the at least one rotor blade, andthe AFC system is configured to modify aerodynamic properties of the atleast one rotor blade. The method includes determining an environmentalcondition surrounding the wind turbine indicative of precipitation, andadjusting at least one of a pitch angle and an azimuth angle of the atleast one rotor blade such that the at least one aperture is wetted.

In yet another aspect, a wind turbine is provided. The wind turbineincludes at least one rotor blade and an active flow control (AFC)system at least partially defined in the at least one rotor blade. TheAFC system is configured to modify aerodynamic properties of the atleast one rotor blade. The wind turbine further includes a sensorconfigured to measure an environmental condition surrounding the windturbine, and a wind turbine controller. The wind turbine controller isconfigured to operate the wind turbine in a first mode, and operate thewind turbine in a second mode different than the first mode depending onthe environmental condition. The second mode includes adjusting at leastone operation parameter of the wind turbine such that fouling of the AFCsystem is reduced.

Further aspects, advantages and features of the embodiments describedherein are apparent from the dependent claims, the description, and theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure including the best mode thereof, to oneof ordinary skill in the art, is set forth more particularly in theremainder of the specification, including reference to the accompanyingfigures.

FIG. 1 is a schematic side view of an exemplary wind turbine.

FIG. 2 is a chord-wise cross sectional view of an exemplary rotor bladethat may be used with the wind turbine shown in FIG. 1.

FIG. 3 is a flowchart of an exemplary method for operating the windturbine shown in FIG. 1.

FIG. 4 is a flowchart of a first alternative method for operating thewind turbine shown in FIG. 1.

FIG. 5 is a flowchart of a second alternative method for operating thewind turbine shown in FIG. 1.

FIG. 6 is a flowchart of a third alternative method for operating thewind turbine shown in FIG. 1.

FIG. 7 is a flowchart of a fourth alternative method for operating thewind turbine shown in FIG. 1.

FIG. 8 is a schematic view of the rotor blade shown in FIG. 2 duringperformance of the method shown in FIG. 7 as coupled to a rotatingrotor.

FIG. 9 is a schematic view of the rotor blade shown in FIG. 2 duringperformance of the method shown in FIG. 7 as coupled to a rotatingrotor.

FIG. 10 is a schematic view of the rotor blade shown in FIG. 2 duringperformance of the method shown in FIG. 7 as coupled to a non-rotatingrotor.

FIG. 11 is a schematic view of the rotor blade shown in FIG. 2 duringperformance of the method shown in FIG. 7 as coupled to a non-rotatingrotor.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the various embodiments, one ormore examples of which are illustrated in each figure. Each example isprovided by way of explanation and is not meant as a limitation. Forexample, features illustrated or described as part of one embodiment canbe used on or in conjunction with other embodiments to yield yet furtherembodiments. It is intended that the present disclosure includes suchmodifications and variations.

Modern wind turbines are designed to produce a maximum amount of energy.However, if the wind speed becomes too large and therefore changes anangle of attack above a certain value, flow separation over wind turbineblades occurs and results in stall. In such a situation, energyproduction by the turbine is reduced. If he flow separation over windturbine blades can be delayed, the design of the wind turbine can befavorably changed, e.g. to increase the production of energy across thewind speed range and/or to change design parameters such as chord. Thiswill eventually result in a considerable increase of wind turbine energyproduction.

Flow separation over wind turbine blades can be delayed by blowing gasout of small apertures arranged at the surface of the rotor blade. Thegas may be fed to the apertures through manifolds within the rotor bladeby a gas supply. Various gases may be used, such as, but not limited toair, nitrogen, or carbon dioxide. In the following, the term “air” willbe exemplarily used without any intention to limit the scope of theappended claims. The gas flow rate through the manifolds and out of theapertures is controlled by the gas supply. Controlling the gas flow rateresults in delayed flow separation, which changes the aerodynamicproperties of the rotor blade. The system, including a gas supply,manifolds, and apertures, actively controls a gas flow out of theapertures of the rotor blade. This system is referred to as an activeflow control (AFC) system.

Although the embodiments described herein are illustrated with respectto a nonzero-net-mass flow control system, it should be understood thatthe systems and methods described herein can also be used with azero-net-mass flow control system.

FIG. 1 shows an exemplary wind turbine 10 having an AFC system 100, asensor 12, and a wind turbine controller 14. FIG. 2 shows a chord-wisecross-sectional view of a rotor blade 16 being equipped with AFC system100. As shown in FIG. 1, wind turbine 10 includes three rotor blades 16,however, wind turbine 10 may include more or less than three rotorblades 16. Rotor blades 16 are mounted on a rotor hub 18 that isconnected to a nacelle 20 fixed on top of a tower 22. FIG. 2 shows aposition of AFC apertures 102 of rotor blade 16. In FIG. 2, rotor blade16 is seen along a span-wise axis of rotor blade 16. In typicalsituations, a wind direction 24 impinges rotor blade 16 at an areaslightly above where a chord line 26 intersects rotor blade 16 at aleading edge 28 of rotor blade 16.

Aerodynamic properties of rotor blade 16 are changed by gas beingejected through apertures 102 defined through a surface 30 of rotorblade 16 on its suction side. Apertures 102 are typically positioned onthe suction side of rotor blade 16 downwind of the airfoil maximumthickness. In FIG. 2 the suction side is an upper side of rotor blade16. According to other embodiments, apertures 102 are positioned on arotor blade section where flow separation mainly occurs.

According to embodiments disclosed herein, blade fouling of wind turbine10, and especially fouling of apertures 102 and manifolds 104 includedinside rotor blade 16, is avoided, or at least reduced, by operatingwind turbine in a preventive manner such that AFC system 100, andespecially manifolds 104 and apertures 102 of AFC system 100 collectless contamination or are not subject to contaminants in the firstplace.

In the embodiment shown in FIG. 1, each rotor blade 16 includes at leastone manifold 104. At a downstream end, manifold 104 is connected to atleast one aperture 102 at surface 30 of rotor blade 16. For reasons ofsimplicity, only one rotor blade 16 with only one manifold 104 and oneaperture 102 is depicted. However, a plurality of manifolds 104,typically of different lengths, can be provided within rotor blade 16.Furthermore, each manifold 104 is connected to a plurality of apertures102. Although depicted only for one rotor blade 16, the other rotorblades 16 include manifolds 104 and apertures 102. At an upstream end,manifold 104 is connected to a gas supply 106 from which gas is suppliedto manifold 104. In this context, the terms “upstream” and “downstream”refer to gas flow directions within AFC system 100. In particular, thedownstream direction is defined to be from gas supply 106 to aperture102. The downstream direction is a direction of gas flow during an AFCmode in which gas is ejected through apertures 102 of rotor blades 16 inorder to improve the aerodynamic properties of rotor blade 16. On theother hand, the upstream direction is defined as a direction fromapertures 102 towards gas supply 106. In the embodiment shown in FIG. 1,gas supply 106 is located inside nacelle 20. According to otherembodiments, gas supply 106 may also be located inside tower 22, insiderotor hub 18, and/or inside rotor blade 16. According to alternativeembodiments, there is provided one gas supply 106 for each rotor blade16. According to some of these embodiments, in each rotor blade 16 thereis provided one gas supply 106.

According to the embodiment of FIG. 1, each manifold 104 is configuredto channel gas and is connected to a valve 108. Each valve 108 isconfigured to block a gas flow to a respective manifold 104 and ispositioned within nacelle 20 of wind turbine 10. Each valve 108 may becontinuously adjusted from completely open to completely closed. It isto be understood that the term “blocking” does not necessarily meancomplete blocking, but may also imply partial blocking of manifolds 104.Valves 108 may thus have a flow control function. If valve 108 is notclosed completely, the gas flow through the remaining valves 108 is notincreased to the same extent as compared to the case when valve 108closes completely. According to some embodiments, valves 108 may bereplaced by other flow control devices which are configured to controlgas flows of manifolds 104. According to other embodiments, valves 108may also be positioned inside rotor hub 18, inside a rotor blade 16, orinside tower 22. The latter arrangement may be used when gas supply 106is located inside tower 22. Manifolds 104 of rotor blades 16 aretypically connected in parallel to gas supply 106 and may be blocked bytheir respective valves 108. An AFC controller 110, which may beincluded in or separate from wind turbine controller 14, controls valves108 and gas supply 106. This control is indicated by arrows in FIG. 1.The foregoing is merely exemplary and should not be construed aslimiting because the present application also encompasses embodimentswithout valves 108.

A gas flow rate through manifolds 104 is controlled by AFC controller110. According to one embodiment, AFC controller 110 controls the gasflow rate through manifolds 104 by changing the gas flow rate of gassupply 106. According to a further embodiment, AFC controller 110controls the gas flow rate through manifolds 104 by blocking, e.g.manifold 104, thereby increasing the gas flow rate through the unblockedmanifolds 104. Thus, an ejection pattern of ejected air is altered andthe aerodynamic properties of rotor blade 16 are varied. For blockingmanifolds 104, AFC controller 110 may use valves 108. Each valve 108 maybe continuously adjusted from completely open to completely closed,blocking the gas flow in the latter case. AFC controller 110 is part ofAFC system 100, which includes manifolds 104, apertures 102, air supply106, and valves 108. According to the embodiments described above, AFCsystem 100, particularly AFC controller 110, are configured to modifythe aerodynamic properties of rotor blades 16 which typically results indelaying a flow separation over rotor blades 16.

In the embodiment shown in FIG. 1, wind turbine controller 14 controlsAFC controller 110, gas supply 106, and a pitch controller 32. Accordingto further embodiments, wind turbine controller 14 further controls ayaw angle and/or a generator speed. Generally, wind turbine controller14 is configured to adjust at least one operation parameter of windturbine 10 based on an environmental condition measured by sensor 12such that fouling of AFC system 100 is reduced. According to theembodiment shown in FIG. 1, AFC controller 110 controls gas supply 106,while wind turbine controller 14 controls AFC controller 110. Accordingto other embodiments, wind turbine controller 14 controls gas supply106. Generally, wind turbine controller 14 may control any operationparameter of wind turbine 10. Operation parameters includes, but are notlimited to including, a rotational speed, a pitch angle of rotor blades16, a yaw angle, and/or a gas flow rate through AFC system 100, inparticular through gas supply 106.

Sensor 12 is configured to measure an environmental conditionsurrounding wind turbine 10. The environmental condition is indicativeof fouling of AFC system 100. The environmental condition is processedby wind turbine controller 14 to determine how to operate wind turbine10. Although depicted at nacelle 20, sensor 12 may also be located in orat tower 22, inside rotor hub 18, inside rotor blade 16, and/or awayfrom wind turbine 10. For example, sensor 12 may be a weather stationinstalled near wind turbine 10 or at least near a wind farm in whichwind turbine 10 is located.

The environmental condition includes, without limitation, an airhumidity, a wind speed, an air temperature, an aerosol concentration, anairborne particle concentration, an airborne contaminants concentration,a concentration of any sort of flora, fauna, and/or any by-productsthereof (e.g. fluff), a wind direction, a rain direction, a frequency ofwind gusts, an atmospheric pressure, a wind chill factor, a cloudheight, a cloud cover, a visibility, a dew point, a migration of birdsand/or other animals, and/or a frequency and/or an amount of animalexcrement, in particular bird droppings. Furthermore, any combination oftwo or more of the foregoing environmental conditions may be used todetermine whether an increased risk for AFC fouling exists.

FIG. 3 illustrates an exemplary method 200 of operating wind turbine 10(shown in FIG. 1). By performing method 200, fouling of rotor blade 16(shown in FIGS. 1 and 2) and/or AFC system 100 (shown in FIG. 1) isfacilitated to be corrected and/or prevented. Method 200 is performed bywind turbine controller 14 (shown in FIG. 1) and/or AFC controller 110(shown in FIG. 1) sending commands and/or instructions to components ofwind turbine 10, such as AFC system 100 and/or any suitable component.Wind turbine controller 14 and/or AFC controller 110 is programmed withcode segments configured to perform method 200. Alternatively, method200 is encoded on a computer-readable medium that is readable by windturbine controller 14 and/or AFC controller 110. In such an embodiment,wind turbine controller 14 and/or AFC controller 110 is configured toread computer-readable medium for performing method 200.

Referring to FIGS. 1-3, method 200 includes operating 202 wind turbine10 in a first mode, such as a normal mode. The term “normal mode” refersto a mode of operating wind turbine 10 and/or AFC system 100 such thatAFC system 100 a drives a flow of fluid to increase lift on at least onerotor blade 16. The normal mode includes normal operation over asubstantially entire power curve; operating when a wind speed is too lowto generate power but wind turbine 10 is prepared to generate power;using constant and variable speed-ranges; operating within a peak shaverrange; operating in an above rated condition; and/or performing a stormcut out. Flow characteristics of the fluid flow during the normal modeare determined empirically and/or are predetermined to achieve optimallift on rotor blade 16 depending on ambient conditions, such as a windspeed and/or a wind direction, precipitation, and/or other atmosphericand/or environmental conditions. At least one flow characteristic of thefluid flow may be adjusted and/or varied during the normal mode, basedon changing ambient conditions and/or operating characteristics of windturbine 10, to facilitate achieving optimal blade lift.

Wind turbine 10 and/or AFC system 100 is operated 202 in the normal modeaccording to a predetermined schedule and/or based on wind conditions.For example, when wind speeds are high, operation 202 of AFC system 100is substantially terminated because increased lift on rotor blade 16 isnot desired in such wind conditions. However, even when active flowcontrol is terminated, enough fluid is discharged from apertures 102 tofacilitate preventing insects and/or other debris from flying intomanifolds 104. Such termination of active flow control is considered tobe part of the normal operating mode.

As wind turbine 10 is operating 202 in the normal mode, wind turbine 10determines 204 at least one environmental condition surrounding windturbine 10. More specifically, wind turbine 10 collects data usingsensor 12 and processes the data to determine 204 the environmentalcondition. Based on the environmental condition, it is determined 206whether to operate wind turbine 10 in the normal mode or in a secondmode, such as a cleaning mode. As used herein, the term “second mode”refers to a mode of operating wind turbine 10 and/or AFC system 100 toachieve an outcome in addition to or different than optimal lift onrotor blade 16. As used herein, the term “optimal lift” refers to liftthat maximizes power production and reduces the cost of powerproduction, such as a lift that is optimized to account for initialcosts of wind turbine 10 and/or a lift that produces a maximized ratioof annual energy capture over initial cost; however, any suitableoptimization scheme can be used to achieve optimal lift. It should beunderstood that wind turbine 10 can be operated in more than two modes.In the exemplary embodiment, the second mode is a mode that isintentionally beneficial for performing a cleaning operation rather thanperforming an operation target, such as capturing energy. During thesecond mode, AFC system 100 is controlled to facilitate removing debrisfrom AFC system 100. The second mode includes at least one cleaning modeto facilitate removing debris from AFC system 100.

By comparing the environmental condition with certain criteria, it isdetermined 206 whether to operate wind turbine 10 in the normal mode orin the second mode. If it is determined 206 to operate in the firstmode, wind turbine 10 continues performing steps 202 and 204. If it isdetermined 206 to operate in the second mode, wind turbine 10 switchesfrom the first mode to operate 208 in the second mode. Based on a timeperiod, an operator's command, sensed environmental conditions, and/orany other suitable criteria, wind turbine 10 operates 208 in the secondmode then returns to operating 202 in the first mode.

FIG. 4 is a flowchart of a first example method 300 for operating windturbine 10 (shown in FIG. 1). Method 300 includes at least some of thesteps of method 200 (shown in FIG. 3) and, as such, similar steps areindicated with similar reference numbers. Referring to FIGS. 1, 2, and4, while wind turbine 10 is operating 302 in the first mode, method 300starts by determining 204 an environmental condition of wind turbine 10indicative of fouling of AFC system 100 by sensing 304 an environmentalcondition using sensor 12. At least one sensed environmental conditionis used to determine 306 specific values that indicate a higher risk offouling of AFC system 100 of wind turbine 10. More specifically,estimated insect density and/or aerosol concentration are determined 306from the sensed environmental condition.

For example, insect density, concentration of dust, impurities, oraerosols or the like are typical indicators that an elevated risk of AFCfouling exists. For example, on days with specific weather conditions,flying insects are likely to fly in the air next to wind turbine 10.Such flying insects may enter manifolds 104 of AFC system 100 viaapertures 102, thus clogging manifolds 104 and apertures 102. Similarly,fouling of AFC system 100 is more likely when there is a highconcentration of dust particles or aerosols in the ambient air. Suchconditions are detected 304 by sensor 12 either directly or indirectly.For example, empirical knowledge or theoretical considerations mayconnect certain weather conditions with high insect activity or highaerosol concentration. Therefore, a higher aerosol concentration may beindirectly determined 306 from the actual weather conditions.

According to another example, sensed environmental conditions can beused to determine 306 insect density. Certain species of insects preferto fly in conditions of high air humidity, low winds, and temperaturesabove 10° C. If all those conditions are fulfilled, probability is highthat apertures 102 may be clogged by those insects.

The estimated insect density value and/or the estimated aerosolconcentration value are determined 306 based on a state of theenvironmental condition sensed in step 304. According to furtherembodiments, additionally or alternatively to the estimated insectdensity value and/or the estimated aerosol concentration value, at leastone of the above mentioned environmental conditions is used in method300.

It is decided 308 whether the estimated insect density value and/or theestimated aerosol concentration value is larger than a respectivethreshold value. The threshold value need not be constant for adifferent environmental condition. The threshold value rather may be acomplex function of various parameters, e.g. air humidity, airtemperature, and/or other variables. In a particular embodiment, atypical value for the insect density threshold is in a range betweenabout 0.003 m⁻³ and about 0.01 m⁻³, and an aerosol concentrationthreshold value is approximately equal to 10 μg·m⁻³. According to someembodiments, the threshold function is influenced by which part of apower curve wind turbine 10 is running, a history of fouling-reductionactions, and/or a history of the environmental condition, e.g. lastcouple of weeks or years. According to yet further embodiments, insteadof the above mentioned step 308, a decision making process isimplemented. In this decision making process, it is decided whether ornot the detected environmental condition, in particular the measuredinsect density or aerosol density, makes it necessary to switch to thesecond mode, such as a fouling-reducing mode.

In the exemplary embodiment, if neither the estimated insect densityvalue nor the estimated aerosol concentration value is larger than itsrespective threshold value, operation of wind turbine 10 is not changedfrom the first mode to the second mode. More specifically, in theexemplary embodiment, the operation parameters of wind turbine 10 arenot changed based on the environmental conditions, and method 300returns to step 302 to continue monitoring of the environmentalcondition surrounding wind turbine 10. It will be understood by thoseskilled in the art that any operation parameter may be adjusted based onconsiderations other than AFC fouling. In this context, energy yieldand/or turbine loads are the most prominent consideration so thatoperation parameters, e.g. pitch angle, may be adjusted in order toincrease the energy yield of the turbine. On the other hand, safetyconsiderations must be observed so that very high wind speeds or faultsin the electrical subsystem may cause a turbine shut-down without anyinsect bloom or dust in the air. Whether energy yield, safetyrequirements, fouling prevention or any other consideration prevails ina certain situation will be determined on the exact circumstances ofthis situation.

In step 308, in the event that the estimated insect density value and/orthe estimated aerosol concentration value is larger than its respectivethreshold value, operation of wind turbine 10 is changed from the firstmode to the second mode. More specifically, in the exemplary embodiment,the second mode includes increasing 310 the gas flow of AFC system 100for blowing insects or impurities out of rotor blade 16 throughapertures 102 and/or for preventing insects or impurities from enteringrotor blade 16 through apertures 102. By blowing insects or impuritiesout of rotor blade 16 through apertures 102, AFC system 100 is cleaned,whereas by preventing insects or impurities from entering rotor blade 16through apertures 102, AFC system 100 is prevented from fouling. Byincreasing 310 the gas flow through AFC system 100, clogging ofapertures 102 of AFC system 100 by insects, aerosols, impurities, dustparticles, dirt, and/or the like is prevented or at least reduced.Furthermore, not only apertures 102, but also manifolds 104, may becleaned by increasing 310 the gas flow through AFC system 100. In thiscontext, it is to be noted that cleaning is typically applied when windturbine 10 is not producing power, whereas fouling prevention could beapplied all the time and/or when demanded by the environmentalcondition.

After step 310, the cleaning of AFC system 100 ends and operation ofwind turbine 10 returns 302 to the first mode. Typically, wind turbine10 will continue to monitor 204 the environmental condition by returningto step 302. According to a further embodiment, an operator may chooseto continue with step 302, to end method 300, and/or to continue with adifferent method.

FIG. 5 is a flowchart of a second example method 400 for operating windturbine 10 (shown in FIG. 1). Method 400 includes at least some of thesteps of method 200 (shown in FIG. 3) and, as such, similar steps areindicated with similar reference numbers. Referring to FIGS. 1, 2, and5, while wind turbine 10 is operating 402 in the first mode, method 400includes determining 204 an environmental condition surrounding windturbine 10 indicative of fouling of AFC system 100. In general, windturbine 10 is operated such that, based on the detected environmentalcondition, fouling of AFC system 100 is reduced.

To determine 204 the environmental condition, at least one environmentalcondition is measured and/or sensed 404 by sensor 12. The measured orsensed value is then processed to determine 406 an estimated insectdensity value and/or an estimated aerosol concentration value.

In step 408, it is then decided whether the estimated insect densityvalue and/or the estimated aerosol concentration value is larger thanits respective threshold value. The threshold value need not be constantfor a different environmental condition. The threshold value rather maybe a complex function of various parameters, e.g. air humidity, airtemperature, and/or other variables. According to some embodiments, thethreshold function is influenced by which part of a power curve windturbine 10 is running, a history of fouling-reduction actions, and/or ahistory of the environmental condition, e.g. last couple of weeks oryears. According to yet further embodiments, instead of the step 408, adecision making process is implemented. In this decision making process,it is decided whether or not the detected environmental condition, inparticular the measured insect density or aerosol density, makes itnecessary to switch to the second mode, such as a fouling-reducing mode.

In the exemplary embodiment, if neither the estimated insect densityvalue nor the estimated aerosol concentration value is larger than itsrespective threshold value, wind turbine 10 continues operating 402 inthe first mode. More specifically, the operation parameters of windturbine 10 are not changed, and method 400 returns to step 402 tocontinue monitoring the environmental condition surrounding wind turbine10. As explained above, considerations other than AFC fouling may resultin adjusting the operational parameters.

In the event that the estimated insect density value or the estimatedaerosol concentration value is larger than its respective thresholdvalue, operation of wind turbine 10 is changed from the first mode tothe second mode, and wind turbine 10 operates 410 in the second mode forsome time. More specifically, in the exemplary embodiment, the secondmode includes shutting down 410 wind turbine 10. According to someembodiments, when shutting down 410 wind turbine 10, rotor blades 10 maybe pitched to a feathered position to avoid insects hitting a region ofblade surface30 where manifolds 104 and/or apertures 102 are located.

After step 410, an operator or wind turbine controller 14 may restartwind turbine 10 to operate 402 in the first mode at some point when boththe insect concentration and the aerosol concentration have droppedbelow their respective threshold values. To this end, the environmentalconditions may be further monitored while wind turbine 10 is shut down410. The operator may also have the option to continue with anothermethod of operating wind turbine 10.

FIG. 6 is a flowchart of a second example method 500 for operating windturbine 10 (shown in FIG. 1). Method 500 includes at least some of thesteps of method 200 (shown in FIG. 3) and, as such, similar steps areindicated with similar reference numbers. Referring to FIGS. 1, 2, and6, while wind turbine 10 is operating 502 in the first mode, method 500includes determining 504 environmental conditions, such as a presence ofprecipitation, using sensor 12. Based on data collected by sensor 12,wind turbine 10 determines 506 whether or not it is precipitating at therespective moment. In the event it is not precipitating, steps 502 and504 are repeated until there is precipitation. Thus method 500 onlycontinues to execute operations in case precipitation is detected.Method 500 is described with respect to detecting rain, however itshould be understood that method 500 may be performed with any suitableprecipitation.

In the event a rain shower is detected in step 506, method 500 includesoperating 208 wind turbine 10 in the second mode rather than the firstmode. More specifically, when it is determined 506 that it is raining,method 500 continues to step 508 where pitch angles of rotor blades 16,an azimuth angle of rotor blade 16, and/or a rotor blade rotationalspeed are adjusted such that apertures 102 of AFC system 100 are wettedby the rain. To this end, rotor blades 16 are rotated such thatapertures 102 face a rain direction relative to rotor blades 16. In thisconnection, “facing” means that an angle between a surface normal ofaperture 102 and the rain direction is smaller than about 90°. In otherwords, rotor blades 16 are positioned such that rain impinges onto rotorblade surface 30 in an area where apertures 102 are located. Typicallyapertures 102 face a similar direction so that the technical meaning ofthe above is easily understood by a person skilled in the art.

In the exemplary embodiment, after wetting of apertures 102, the rainwater is pulled 510 into rotor blades 16, more particularly intomanifolds 104 formed within rotor blades 16, through apertures 102. Thisis typically accomplished by reversing a gas flow direction of AFCsystem 100 to the upstream direction. In other words, the gas flows intoapertures 102 and towards gas supply 106. As described above, this may,for example, be achieved by reversing a pumping direction and/or arotation direction of a fan or similar device. To further increase thereverse gas flow, in some embodiments, a pitch angle of each rotor blade16 is adjusted such that a static pressure at apertures 102 is increasedto force air to flow into apertures 102. Generally, before this is done,the AFC mode is stopped or, even better, an inward gas flow intoapertures 102 is initiated. Thus, the reversed operation of AFC gassupply 106 and the static pressure cooperate in order to increase thegas flow into apertures 102. According to other embodiments, instead ofactively pulling 510 water into rotor blade 16 through apertures 102,water enters apertures 102 and manifolds 104 due to gravity, cavitation,and/or capillary forces.

In step 512, the rain water and impurities within manifolds 104 andapertures 102 are blown out of rotor blade 16. To this end, the gas flowdirection of AFC system 100 is again reversed to a downstream flow inwhich the gas flows from gas supply 106 towards apertures 102. In someembodiments, this downstream gas flow is assisted by adjusting the pitchangle of each rotor blade 16 and/or an azimuth angle of rotor blade 16such that a dynamic pressure at apertures 102 is increased to pull thegas out of apertures 102. By blowing rain water and impurities out ofrotor blade 16, manifolds 104 and apertures 102 are cleaned and, thus,fouling of AFC system 100 is reduced. After cleaning of rotor blades 16,method 500 may continue at step 208, or may return to step 502.

FIG. 7 is a flowchart of a second example method 600 for operating windturbine 10 (shown in FIG. 1). Method 600 includes at least some of thesteps of method 200 (shown in FIG. 3) and, as such, similar steps areindicated with similar reference numbers. Referring to FIGS. 1, 2, and7, while wind turbine 10 is operating 602 in the first mode, method 600includes determining 604 environmental conditions, such as a presence ofprecipitation, using sensor 12. Based on data collected by sensor 12,wind turbine 10 determines 606 whether or not it is precipitating, suchas raining, at the respective moment. These detection steps 604 and 606are looped until precipitation is detected. Thus method 600 onlycontinues to execute operations in case of precipitation. Method 600 isdescribed below with respect to detecting rain, however it should beunderstood that method 600 may be performed with any suitableprecipitation.

In the event of a rain shower, method 600 includes operating 208 windturbine 10 in the second mode rather than the first mode. Morespecifically, in the exemplary embodiment, method 600 continues to step608 where pitch angles of rotor blades 16, an azimuth angle of rotorblade 16, and/or a rotor blade rotational speed are adjusted such thatapertures 102 are wetted. To this end, rotor blades 16 are rotated suchthat apertures 102 face a rain direction. According to otherembodiments, rotor blades 16 are rotated such that apertures 102 facethe wind direction.

In the exemplary embodiment, after step 608, wind turbine 10 is operated610 at such a low rotational speed that spinning off of water from rotorblades 16 is reduced. Typically, for wind turbine 10 having bladelengths of about 50 meters, and a tower height of about 16 meters, thishappens at a rotational speed between about 5 rotations per minute (rpm)and about 15 rpm. Thus, more rain water remains on rotor blades 16 andis available for dissolving and/or suspending contaminants and/or forrinsing apertures 102 and manifolds 104 of AFC system 100.

The next step of the method may be step 612 or step 614 or a combinationof both applied simultaneously or sequentially. In step 612, a gas flowdirection of gas supply 106 is reversed with respect to a direction ofgas flow during an AFC mode. In other words, the gas flow direction isnow upstream so that air and water at or near apertures 102 are pulled616 into rotor blade 16 from blade surface 30. Thus, apertures 102and/or manifolds 104 of AFC system 100 are rinsed with rain water.

In the exemplary embodiment, in particular, AFC controller 110 and gassupply 106 are configured to reverse 612 the gas flow direction. Inother words, AFC system 100 is configured to switch to an upstream gasflow, in contrast to the downstream gas flow during the AFC mode. Forexample, this may be achieved by reversing 612 the pumping direction ifgas supply 106 includes a pump, or by reversing 612 the rotationaldirection if gas supply 106 includes a fan or similar device. It will beunderstood by those skilled in the art that the foregoing is only meantas non-limiting examples.

To assist this reversed gas flow, a pitch angle of rotor blades 16 maybe adjusted 614 such that an actual angle of attack α is about 270°. Insome embodiments, especially where the gas flow direction of gas supply106 cannot be reversed, step 614 is applied alternatively to step 375.The situation in step 614 is shown with more detail in FIGS. 8 and 9 fora rotating rotor blade and in FIGS. 10 and 11 for a non-rotating rotorblade. Therein, angle of attack α is defined as an angle between chordline 26 and a velocity vector v_(rel) representing a relative motionbetween rotor blade 16 and inflow. For the non-rotating rotor bladeshown in FIGS. 10 and 11, vector v_(rel) equals an ambient wind vectorin direction and magnitude. For a rotating rotor blade, which case isdepicted in FIGS. 8 and 9, the relative wind velocity v_(rel) isdetermined as a vector sum of v_(∞) being the ambient wind speed vectorand the rotational speed vector v_(x) of rotor blade 16. In thiscontext, it is observed that rotational velocities of rotor blades 16may be larger than typical wind speeds. This vector sum is donegraphically within FIGS. 8 and 9. According to other embodiments, instep 614, actual angle of attack α is chosen in q range between about18° and about 330°. In this range, air is forced to flow or pulled 616into apertures 102.

FIGS. 9 and 11 show situations where actual angle of attack α is about270°. In these situations, air is forced to flow or is pulled 616 intoapertures 102. This can be seen very easily for the case of anon-rotating rotor blade 16 because, in that case, wind blows directlyinto apertures 102 on the suction side of rotor blade 16, as depicted inFIG. 11. FIG. 9 shows the case for a rotating rotor blade. According toother embodiments, actual angle of attack α is in the range betweenabout 18° and about 330°. In this range, air is forced to flow or ispulled 616 into apertures 102.

On the other hand, FIGS. 8 and 10 show situations for a rotating rotorblade and a non-rotating rotor blade, respectively, when an outward,i.e. downstream, gas flow is promoted. In this event, angle of attack αis set to a value smaller than a stall angle. The stall angle is definedas an angle at which stalling of a section of rotor blade 16, i.e.complete flow separation, occurs.

Referring again to FIG. 7, after reversing the gas flow direction insteps 612 and 614, rain water is pulled 616 into manifolds 104 throughapertures 102. In the following, apertures 102 and/or manifolds 104 ofAFC system 100 are rinsed with the rain water. After the rain water hasdissolved and loosened the impurities within apertures 102 and manifolds104, the impurities are rinsed or blown out of rotor blade 16.

To this end, ending of the rain shower may be awaited in step 618.Subsequently, a rotational speed of wind turbine 10 is increased in step620. By increasing 620 the rotational speed of wind turbine 10, thewater and impurities on or inside rotor blade 16 are thrown out or spunoff rotor blade 16 thus cleaning rotor blade 16, and in particular AFCsystem 100. It will be understood by those skilled in the art steps 618and/or 620 are optional and either or both step may be omitted.

After the optional steps 618 and 620, method 600 continues with combinedor alternative steps 622 and 624. This means the next step of method 600may be step 622 or step 624 or a combination of both appliedsimultaneously or sequentially. Typically both steps 622 and 624 areexecuted simultaneously or consecutively in order to amplify theireffect.

In step 622, the gas flow direction of AFC system 100 is again reversedand set to the downstream direction. In that case, the gas flows fromgas supply 106 to apertures 102. This promotes gas to flow out ofapertures 102. To assist this outward gas flow, the pitch angles ofrotor blades 16 may be adjusted such that actual angle of attack α issmaller than a stall angle, i.e. an angle of attack where stallingoccurs.

FIGS. 8 and 10 show situations where actual angle of attack α is smallerthan the stall angle for the case of a rotating rotor blade and anon-rotating rotor blade, respectively. At the stall angle, rotor blade16 has maximum lift. If actual angle of attack α becomes larger than thestall angle, wind turbine 10 generates less power and the unsteadinessin the aerodynamic response of a given airfoil is increased. Botheffects are undesirable. In case angle of attack α is smaller than thestall angle, the flow at the given blade section is substantiallyattached. As the position of apertures 102 is on the suction side afterthe chord-wise location of maximum airfoil thickness of rotor blade 16,the gas flow out of apertures 102 will be promoted.

According to method 600, in the next step 626, the rain water togetherwith the contaminants, e.g. insects or dirt, contained within manifolds104 and apertures 102 are blown out due to the gas flow. By blowing out626 rain water and impurities, manifolds 104 and/or apertures 102 of AFCsystem 100 are cleaned thus reducing fouling of AFC system 100. Afterrain water is blown out 626 of rotor blade 16, the operation of windturbine 10 continues with step 208 or returns to step 602.

A technical effect of the system and methods described herein includesat least one of: (a) operating a wind turbine in a first mode; (b)determining an environmental condition surrounding a wind turbineindicative of fouling of an AFC system; (c) operating a wind turbine ina second mode different than a first mode based on an environmentalcondition, wherein the second mode facilitates reducing fouling of anAFC system; (d) determining an environmental condition surrounding awind turbine indicative of precipitation; and (e) adjusting at least oneof a pitch angle and an azimuth angle of at least one rotor blade suchthat at least one aperture is wetted.

This written description uses examples, including the best mode, toenable any person skilled in the art to make and use the describedsubject-matter. While various specific embodiments have been disclosedin the foregoing, those skilled in the art will recognize that thespirit and scope of the claims allows for equally effectivemodifications. Especially, mutually non-exclusive features of theembodiments described above may be combined with each other. Thepatentable scope is defined by the claims, and may include suchmodifications and other examples that occur to those skilled in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal language of theclaims.

1. A method of operating a wind turbine having at least one rotor bladeand an active flow control (AFC) system, the at least one rotor bladehaving at least one aperture defined through a surface thereof and theAFC system configured to modify aerodynamic properties of the at leastone rotor blade by ejecting gas through the at least one aperture, saidmethod comprising: operating the wind turbine in a first mode;determining an environmental condition surrounding the wind turbineindicative of fouling of the AFC system; and operating the wind turbinein a second mode different than the first mode based on theenvironmental condition, the second mode facilitating reducing foulingof the AFC system.
 2. The method according to claim 1, whereindetermining an environmental condition comprising determining at leastone of an air humidity, a wind speed, an air temperature, an aerosolconcentration, an airborne particle concentration, an airbornecontaminants concentration, a wind direction, a rain direction, afrequency of wind gusts, an atmospheric pressure, a wind chill factor, acloud height, a cloud cover, a visibility, and a dew point.
 3. Themethod according to claim 1, wherein determining an environmentalcondition comprises determining whether an estimated insect densityvalue is above an insect density threshold value based on theenvironmental condition; and operating the wind turbine in a second modecomprises adjusting at least one operation parameter of the wind turbinebased on the estimated insect density value such that clogging of the atleast one aperture is reduced.
 4. The method according to claim 3,wherein the at least one operation parameter is a gas flow rate of theAFC system, said method further comprising increasing the gas flow rateof the AFC system when the estimated insect density value is larger thanan insect density threshold value.
 5. The method according to claim 3,operating the wind turbine in a second mode comprises shutting down thewind turbine when the estimated insect density value is larger than aninsect density threshold value.
 6. The method according to claim 1,wherein determining an environmental condition comprises determiningwhether an aerosol concentration value is above an aerosol concentrationthreshold value; and operating the wind turbine in a second modecomprises increasing a gas flow rate of the AFC system when the aerosolconcentration value is larger than an aerosol concentration thresholdvalue.
 7. The method according to claim 1, wherein determining anenvironmental condition comprises determining whether it isprecipitating; and operating the wind turbine in a second mode comprisespositioning the at least one rotor blade such that the at least oneaperture is wetted by the precipitation.
 8. The method according toclaim 7, wherein the positioning comprises at least one of: adjusting apitch angle of the at least one rotor blade; and adjusting an azimuthangle of the at least one rotor blade.
 9. The method according to claim7, wherein the positioning comprises positioning the at least one rotorblade relative to a rain direction such that the at least one aperturefaces the rain direction.
 10. A method of operating a wind turbinehaving at least one rotor blade and an active flow control (AFC) system,the at least one rotor blade including at least one aperture definedthrough a surface of the at least one rotor blade and the AFC systemconfigured to modify aerodynamic properties of the at least one rotorblade, said method comprising: determining an environmental conditionsurrounding the wind turbine indicative of precipitation; and adjustingat least one of a pitch angle and an azimuth angle of the at least onerotor blade such that the at least one aperture is wetted.
 11. Themethod according to claim 10, further comprising operating the windturbine at a rotational speed that prevents the precipitation fromspinning off of the at least one rotor blade.
 12. The method accordingto claim 10, wherein at least one manifold is defined in the at leastone rotor blade, said method further comprising: initiating an inwardgas flow into the at least one manifold through the at least oneaperture; and adjusting the pitch angle of the at least one rotor bladeto increase a static pressure at the at least one aperture to promoteair to flow into the at least one aperture.
 13. The method according toclaim 10, further comprising adjusting the pitch angle of the at leastone rotor blade to reduce a static pressure at the at least one apertureto promote gas to flow out of the at least one aperture.
 14. The methodaccording to claim 10, further comprising adjusting the pitch angle ofthe at least one rotor blade such that an actual angle of attack of theat least one rotor blade is smaller than a stall angle.
 15. The methodaccording to claim 10, wherein at least one manifold is defined in theat least one rotor blade, said method further comprising: initiating aninward gas flow into the at least one manifold; and adjusting the pitchangle of the at least one rotor blade such that an actual angle ofattack of the at least one rotor blade is about 270 degrees.
 16. Themethod according to claim 10, further comprising: setting a gas flowdirection of the AFC system to an upstream gas flow direction; andpulling rain water into the at least one aperture.
 17. The methodaccording to claim 16, further comprising: after pulling the rain waterinto the at least one aperture, setting the gas flow direction of theAFC system to a downstream gas flow direction; and blowing gas throughthe at least one aperture to remove the rain water and impurities in therain water from inside the AFC system.
 18. A wind turbine comprising: atleast one rotor blade; an active flow control (AFC) system at leastpartially defined in said at least one rotor blade, said AFC systemconfigured to modify aerodynamic properties of said at least one rotorblade; a sensor configured to measure an environmental conditionsurrounding said wind turbine; and a wind turbine controller configuredto: operate said wind turbine in a first mode; and operate said windturbine in a second mode different than the first mode depending on theenvironmental condition, the second mode including adjusting at leastone operation parameter of said wind turbine such that fouling of saidAFC system is reduced.
 19. The wind turbine according to claim 16,wherein said sensor is a rain sensor.
 20. The wind turbine according toclaim 16, wherein said AFC system is configured to reverse a gas flowdirection through said AFC system.